1. Field of the Invention
The present invention relates to a surface acoustic wave device having an electrode structure that can withstand high power at a high frequency band and a method of manufacturing the same.
2. Description of the Related Art
Surface acoustic wave devices are electronic components using surface acoustic waves which are propagated in a state in which mechanical vibration energy is concentrated around only surfaces of solid substances, and are used for filters, resonators, duplexers, etc.
In recent years, the trend in mobile communication terminals, such as mobile phones, are rapidly becoming smaller in size and lighter. For this reason, electronic components mounted on such mobile communication terminals should also be reduced in size.
A surface acoustic wave device has a configuration in which a pair of interdigital transducers (IDT electrodes) made of a conductive material is disposed on a piezoelectric substrate such that finger portions of the interdigital transducers are alternately arranged. The surface acoustic wave device having such a simple structure is suitable for filters, resonators, and duplexers that should be reduced in size when used for such mobile communication terminals.
In addition, when the surface acoustic wave device is used as a resonator, it is important that the variations in the serial resonance frequency and the parallel resonance frequency should be small when the device temperature changes.
In the related art, it has been known that the interdigital transducers and the piezoelectric substrate are covered with a silicon oxide layer so as to reduce the variations in the serial resonance frequency and the parallel resonance frequency due to the temperature change of the surface acoustic wave device.
However, in the case of the configuration in which the interdigital transducers and the piezoelectric substrate are covered with the silicon oxide layer, a problem occurs in that voids are generated in the interdigital transducers.
FIG. 11 is a plan view illustrating a conventional surface acoustic wave device. The surface acoustic wave device includes a piezoelectric substrate 12; and an interdigital transducer portion 13 and an interdigital transducer portion 14 formed on the piezoelectric substrate 12. In addition, the interdigital transducer portions 13 and 14 and the piezoelectric substrate 12 are covered with a silicon oxide layer, but the silicon oxide layer is not shown in FIG. 11. The interdigital transducer portions 13 and 14 are electrically connected to connection electrode portions 15 and 16 that are provided to connect the surface acoustic wave device to an external circuit. FIG. 12 is a cross-sectional view of the interdigital transducer portions 13 and 14 taken along the single-dotted chain line of FIG. 11, as viewed from the direction indicated by arrows. The interdigital transducer portions 13 and 14 and the piezoelectric substrate 12 are covered with a silicon oxide layer 10.
As shown in FIGS. 11 and 12, voids B are generated in the interdigital transducer portions 13 and 14. The voids B are generated because a material of the interdigital transducer portions 13 and 14 is lost due to thermal stress during a process in which the interdigital transducer portions 13 and 14 and the piezoelectric substrate 12 are coated with silicon compound and are then subjected to a thermal treatment under atmosphere of an oxygen and a moisture.